The present invention relates to a multicolor image recorder using toner of a plurality of colors as a developer and, more particularly, to a device applicable to such an image recorder for controlling the density of toner.
A recent achievement in the realm of imaging art is a multicolor electrophotographic copier having a plurality of developing units which are arranged around a photoconductive element and each stores toner of a different color. In this kind of copier, one of the developing units which is designated by an operator is actuated to develop an electostatic latent image provided on the photoconductive element by the toner of particular color which is stored in that unit. Specifically, a first and a second developing unit which store, for example, red toner and black toner, respectively, are disposed around a photoconductive element in such a manner as to be movable toward and away from the latter. As an operator manipulates a mode switch provided on an operation board to select a particular mode, any of the first and second developing units which is associated with that mode is brought close to the surface of the photoconductive element. Then, a sequence of copying steps known in the art, i.e., charging, exposing, developing and transferring steps are effected to produce a copy in a desired color.
In the case that each of the first and second developing units stores a two-component developer which is made up of toner and carrier, the mixture ratio of toner and carrier has to be maintained constant in order to uniformize the density of reproduced images by controlling the density of toner. An implementation heretofore adopted for the toner density control is such that an exclusive density pattern is produced by particular toner in a non-image area of the photoconductive element at a predetermined period, the density of such a pattern is sensed in terms of an amount of reflected light by a reflection type sensor which is constituted by a light emitting element (LED) and a light sensitive element (P sensor) as well known in the art, and the amount of reflected light sensed is converted into an electric signal. More specifically, the toner density is sensed by comparing an output of the P sensor which is associated with those areas before and after the density pattern which have been erased (substantially constant) and an output of the P sensor which is associated with the density pattern (variable with toner density). As regards the period for sensing toner density, the sensing operation may be effected with a copy produced first after the turn-on of a main switch and every tenth copy which follows the first copy. When it is decided that toner density is short, toner is continuously supplied to a particular developing unit copy by copy until the next timing for sensing toner density is reached, e.g., up to the tenth copy. Since the light to which the P sensor is responsive is selected to be infrared light, the toner density control is accomplished with no regard to the color of toner despite that the detection level of the P sensor is variable. Hence, identical optical sensors are individually associated with toner of different colors or, alternatively, a single optical sensor is shared by toner of different colors.
A problem with the prior art electrophotographic copier described above stems from the fact that the first developing unit storing red toner and the second developing unit storing black toner which is used more frequently than red toner have, in many cases, different capacity from each other. Should the toner density associated with the individual developing units be sensed at the same period, the toner density associated with the second developing unit would be scattered far more significantly than that associated with the first developing unit rendering the density of reproduced images unstable.
Furthermore, the deterioration of red toner stored in the first or smaller developing unit due to aging is apt to occur at a higher rate than that of black toner which is stored in the second or larger developing unit. It follows that, when identical optical sensors are used to control the density of the toner of different colors, the red toner is determined to be short due to aging even when its density is appropriate. The result is an excessive supply of red toner which would bring about smears in the background area of a copy as well as mechanical troubles ascribable to toner being scattered around.